The most recent issue of the Mayo Clinic Health Letter reports on the complicated relationship between calories and weight loss.

While burning 500 calories a day should result in one pound of weight loss per week, as one pound equals 3,500 calories, factors such as body composition, metabolism and brain function can affect the formula, according to the clinic.

It happens because dieters lose not only fat, but also water and lean tissue as they exercise.

Eating all of your calories for the day at once, for example, reduces the rate of metabolism, which makes it harder to lose weight.

On the other hand, calories from moderate amounts of alcohol or nuts have been found not to add weight. Sugar substitutes also have few calories, but researchers were puzzled to find that people who drink diet soda tend to weigh more than those who don’t.